1. Field of Invention
This invention relates to photothermographic materials and in particular, it relates to post-processing stabilization of photothermographic silver-containing materials.
2. Background to the Art
Silver halide-containing photothermographic imaging materials processed with heat, and without liquid development, have been known in the art for many years. These materials generally comprise a support having thereon light-sensitive silver halide; a light-insensitive, organic silver salt; and a reducing agent for the light-insensitive organic silver salt.
The light-sensitive silver halide is in catalytic proximity to the light-insensitive organic silver salt so that the latent image, formed by irradiation of the silver halide, serves as a catalyst for the oxidation-reduction reaction of the light-insensitive, organic silver salt with the reducing agent when the emulsion is heated above about 80.degree. C. Such media are disclosed, for example, in U.S. Pat. Nos. 3,457,075; 3,839,049; and 4,260,677.
A variety of ingredients may be added to these basic components to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic emulsions, as disclosed in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are disclosed in U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
A common problem that exists with photothermographic systems is post-processing instability of the image. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver during room light handling. Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate post-processing image stabilizers has been used to impart post-processing stability. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure, June, 1978, item 17029. U.S. Pat. Nos. 4,245,033; 4,837,141; and 4,451,561 describe sulfur compounds that are development restrainers for photothermographic systems. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are disclosed in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles such as 3-amino-5-benzothio- 1,2,4-triazole as post-processing stabilizers are disclosed in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure, May 1978, items 16977 and 16979.
Problems arising from the addition of stabilizers may include thermal fogging during processing and losses in photographic speed and maximum density or contrast at effective stabilizer concentrations.
The most effective traditional antifoggant has been mercuric ion as disclosed, for example, in U.S. Pat. No. 3,589,903. However, mercury compounds are environmentally undesirable and there is a need to find equally effective, but less hazardous antifoggants.
Various compounds have been used as antifoggants in place of mercury compounds in photothermographic silver imaging systems.
U.S. Pat. Nos. 3,707,377 and 4,108,665 disclose stabilization of thermally processed silver materials against formation of printout silver using a small amount of halogenated organic material, such as tribromoethanol. It is believed that photodecomposition of the halogenated organic compound generates bromine atoms, which combine with silver atoms from printout silver halide to maintain a low silver printout until the halogenated material is exhausted. The process can take years, though, depending on the amount of light exposure and the amount of halogenated material in the coating.
U.S. Pat. No. 3,764,329 uses N-bromoacetamide, an antifoggant, which has the oxidizing power of a halogen compound to bleach the latent image until it is time to use the light-sensitive material. The light sensitivity is achieved by brief heating to destroy the N-bromoacetamide used.
U.S. Pat. No. 3,874,946 describes a stabilizer precursor comprising a compound, which is a photolytically active polybrominated organic compound comprising: (a) a 2,4-bis(tribromomethyl)-s-triazine, (b) a polybromoalkylsulfonyl compound having an aromatic chromophore group having the ability to absorb electromagnetic radiation with a wavelength between about 250 and 385 nm, or (c) combinations thereof in a photothermographic element or composition. A preferred stabilizer disclosed in this Patent is 2-tribromomethylsulfonyl-benzothiazole (2-TBMBz).
U.S. Pat. No. 4,546,075 discloses, as antifoggants in place of mercury. compounds, the use of trihalomethyltetrazole derivatives, trihalomethyl-benzimidazoles, and corresponding benzoxazoles and benzthiazoles.
Japanese published Pat. Appl. No. 59-57234 discloses, as antifoggants in place of mercury compounds in photothermographic silver systems, the use of compounds of the formula: EQU R.sup.a --CX.sub.2 --R.sup.b
wherein: X represents halogen, preferably Br, and R.sup.a and R.sup.b are optionally substituted acyl, oxycarbonyl, oxysulfonyl, alkylsulfonyl, arylsulfonyl, aralkylsulfonyl, carboxyl, sulfo, or sulfamoyl.
U.S. Pat. No. 4,465,761 discloses the use of organohalogen compounds, including compounds of the formula: EQU R.sup.c --SO.sub.2 --CX.sub.3
wherein: X represents halogen and R.sup.c is an aryl group, an alkyl group, allyl, or a heterocyclic residue. The materials are used in a non-silver photosensitive composition in which the organohalogen compounds are capable of forming a free-radical by the irradiation with light.
U.S. Pat. No. 4,452,885 discloses, as antifoggants in place of mercury compounds, the use of 2-trihalomethyloxazole derivatives.
U.S. Pat. No. 4,756,999 discloses as antifoggants in place of mercury compounds, the use of various heterocyclic systems bearing trihalomethyl functionality.